Tire Pressure Monitoring Systems (TPMS) play an important role in vehicle safety and emissions reduction. A majority of this market is served by direct tire pressure monitoring systems, in which each tire contains a TPMS sensor module. Thus, a battery powered sensor module is assembled in the inside of a tire to monitor a tire pressure thereof. The sensor module contains a pressure sensor, a microcontroller, an radio-frequency (RF) transmitter and a coin battery cell.
Principally, the sensor module measures the tire pressure and uses a unidirectional link to transmit the measurement data to a central unit in the vehicle. Since battery cannot be changed, sensor module lifetime is determined by battery lifetime. A major portion of the power consumption is generated by the RF transmission. Hence, it is an important task to reduce power consumption for RF transmission as much as possible.
The sensor module may also have a low-frequency (LF) receiver, which is used to configure the sensor module after mounting sensor module to the tire in the car production or in a repair garage (e.g., in case of replacement modules or firmware update for maintenance of sensor modules which are already in use). Typically, the downlink communication from the module to the car is implemented via the RF transmitter at 315 or 434 MHz, whereas the uplink communication to the module is implemented via the LF receiver at 125 kHz. Thus, two communication devices with two communication channels are used.
In order to properly assess each tire, a vehicle must be able to localize each TPMS sensor module, meaning it must be able to know which module is located at which tire position (e.g., front left, rear right, etc.). Today, the most cost efficient and also most used solution for localizing a TPMS sensor module is using a correlation of two types of data.
The first type of data is an angle of the TPMS module during rotation. The TPMS module is able to either transmit data at a fixed angle during rotation or, on the other side, at least inform a TPMS electronic control unit (ECU) at which time it has been at which rotational angle related to the time of the RF transmission. With these two information, a phase of the TPMS sensor module and a time of that phase, the ECU can determine the angle of any of the four TPMS sensor modules during rotation.
The second type of data is anti-lock braking system (ABS) data. Typically, an ABS ECU puts the current ABS counter values, counting from [0 . . . a] during each rotation, on a Controller Area Network (CAN) bus. The TPMS ECU can then correlate the two types of data in order to determine which TPMS module is located at which tire position.
This solution has some disadvantages. One disadvantage is that the TPMS sensor module must transmit several times during localization process. Thus, it has a negative impact on the charge consumption of the TPMS module, degrading the lifetime of the sensor module. Another disadvantage is that the TPMS ECU must have access to the CAN bus and must also run a rather complex algorithm and provide a big buffer for the data for correlation. A final disadvantage is that some ABS systems don't offer these counter values, but rather a value only corresponding to wheel speed, which cannot be used for this localizing approach. Thus, not all vehicles can use this localizing approach.